DNS, the system that implements domain naming on the Internet, is hierarchical. At the top of the hierarchy, to the right of "pcmag.com" and other domains, is the root domain. It is the ultimate authority for naming.

It's often said that there are 13 root servers, named A through M, but as the blog says, this is an oversimplification for convenience. It would be more accurate to say that there are 13 root server addresses, and that each address may correspond to multiple computers. In fact, there are over 130 servers. Click here for a list of the 13 named root servers, where they are, who runs them, and other important details. Click here for a Google map showing their physical locations.